It is customary to use dam bars on lead frames for molded semiconductor device packages, particularly integrated circuit packages. The dam bars are used to simplify sealing of the package mold on the various finger leads of the frame. The mold is sealed by simply clamping opposed flat faces of the mold onto the dam bar area of the lead frame. The dam bars thus prevent leakage of molding compound out of the die cavity along the fingers of the lead frame. They also provide lateral support for the finger leads during molding. After molding, the dam bars are cut away from their contiguous finger leads to provide discrete finger leads on the molded package.
The molding compound used for these packages is normally rather abrasive in character. For example, a typically used plastic molding composition ordinarily contains 50% or more of an abrasive type of filler such as silicon dioxide, talc, etc. If the dam bars of prior lead frames are spaced too close to the molded package, their cutting die may scrape the edge of the molded package, and become dull. As a result, it is customary to space the dam bars slightly away from the molded package. On the other hand, spacing the dam bars away from the package generates another problem. The spaced dam bars allow a thin web of molding compound to form between the finger leads. This web is popularly referred to as a flash or fin. When the dam bars are cut away the cutting die contacts the web. In addition, portions of the web fall into the cutting die assembly, to eventually accumulate and cause accelerated cutting die wear. In addition, the web may not be completely uniformly removed from the package edge when the dam bars are cut. It may be necessary to complete their removal in a subsequent finishing operation, which adds a step and further cost to package manufacture.
The final removal of the web is most conveniently done under high volume production conditions in a manner that does not produce a smooth surface on the package when the web was removed. There is belief by some that the unfinished edge on the package, where the web was broken away, provides a site where impurities, moisture, and the like, can possibly enter the package and degrade it. There may be no merit to this belief. Nonetheless, it is generally conceded that a smooth finished surface over all of the package is desirable if it can be conveniently produced. We have now discovered a new lead frame configuration that permits one to avoid forming the web without generating the aforementioned dam bar cutting die problems. Hence, if desired, a finished surface can be conveniently produced on the package body even between the finger leads.